Optical disc and fabrication method thereof

ABSTRACT

On a replica substrate  1   a , a reflection layer  11 , a first dielectric layer  12  made of ZnS—SiO 2 , a recording layer  13  made of a phase change type recording material, and a second dielectric layer  14  made of ZnS—SiO 2  are successively formed. In addition, a reaction protection layer  15  made of Si 3 N 4  or SiO 2  is formed on the second dielectric layer  14 . As a result, an information signal portion  1   c  is composed. A light transmissivity sheet is formed through an adhesive layer so that the light transmissivity sheet coats an information signal portion  1   c . As a result, a light transmission layer is composed. When the reaction protection layer  15  is not formed, a reaction protection resin layer made of an ultraviolet ray setting resin is formed so that the reaction protection resin layer coats the information signal portion  1   c.

TECHNICAL FIELD

The present invention relates to an optical disc and a fabricationmethod thereof, in particular, to an optical disc of which a lighttransmission layer formed on a disc substrate is composed of a lighttransmissivity sheet and an adhesion layer.

BACKGROUND ART

In recent years, in the information recording field, a variety ofattempts of research and development have be made for the informationrecording system. In the optical information recording system, data canbe recorded and reproduced contact-freely. As one advantage, the opticalinformation recording system can accomplish a recording density morethan ten times higher than the conventional magnetic recording system.As another advantage, the optical information recording system can applyto individual memory types such as the read-only type, the write-oncetype, the rewritable type, and so forth. Thus, as a system thataccomplishes inexpensive and large-capacity files, it has beenconsidered that the optical information recording system will be widelyused from industries to homes.

Among those, digital audio discs (DAD) and optical video discs, whichare optical discs of read-only memory type, have become common.

In an optical disc such as a digital audio disc, a reflection film thatis a metal thin film such as an aluminum (Al) film is formed on anoptical disc substrate that is a transparent substrate on which anuneven pattern such as pits and grooves that represent an informationsignal is formed. In addition, a protection film that protects thereflection film against moisture (H₂O) and oxygen (O₂) in the atmosphereis formed on the reflection film. When an information signal isreproduced from the optical disc, read light such as laser light isradiated from the optical disc substrate side to the uneven pattern.Corresponding to the difference of reflectance of the incident light andthe reflected light of the read light, the information signal isdetected.

When such an optical disc is produced, an optical disc substrate havingan uneven pattern is formed by the injection molding method. Thereafter,a reflection film made of a metal thin film is formed on the opticaldisc substrate by the vacuum evaporation method. Thereafter, anultraviolet ray setting resin is coated on the reflection film. As aresult, the protection film has been formed.

In the aforementioned optical information recording system, in recentyears, an increase of recording density has been required. To satisfythat, a technology for decreasing the diameter of the spot of the readlight with an increased numerical aperture (NA) of an objective lensthrough which the read light is radiated from an optical pickup has beenproposed. According to the technology, while the NA of an objective lensused to reproduce data from a conventional digital audio disc is 0.45,the NA of an objective lens used to reproduce data from an optical videodisc such as a DVD (Digital Versatile Disc) that has a recordingcapacity six to eight times higher than that of the digital audio discis around 0.60. As a result, the diameter of the spot is decreased.

As the NA of the objective lens is increased, the disc substrate of theoptical disc should be thinned so as to transmit the radiated readlight. This is because the tolerable amount of the deviation angle (tiltangle) of the optical axis of the optical pickup against the verticalline of the disc surface becomes small. The tilt angle is subject toinfluences of aberration and double refraction due to the thickness ofthe substrate. Thus, the substrate is thinned so that the tilt anglebecomes as small as possible. For example, the thickness of thesubstrate of the aforementioned digital audio disc is around 1.2 mm. Incontrast, the thickness of the substrate of an optical video disc suchas a DVD, which has a recording capacity six to eight times higher thanthe digital audio disc, is around 0.6 mm.

However, in consideration of the increasing needs of high recordingdensity, the substrate should be further thinned. To satisfy that,another type optical disc has been proposed. In this optical disc, anuneven pattern is formed on one main plane of a substrate as aninformation signal portion. A reflection film and a light transmissionlayer, which is a thin film that transmits light, are successivelylayered. Read light is radiated from the light transmission layer side.As a result, an information signal is reproduced. In such an opticaldisc, of which read light is radiated from the light transmission layerside and thereby an information signal is reproduced, the film thicknessof the light transmission layer is decreased so that the optical disccan conform with a large NA of an objective lens.

However, when the light transmission layer is thinned, it becomesdifficult to form a light transmission layer with a thermoplastic resinby the injection molding method, which is used in a conventional opticaldisc producing method. In other words, in the conventional technology,it is very difficult to form a light transmission layer that has athickness of around 0.1 mm and that has small double refraction and goodtransparency.

To solve such a problem, a method for forming a light transmission layerwith an ultraviolet ray setting resin. In this method, however, when thelight transmission layer is formed with the ultraviolet ray settingresin, it is very difficult to keep the film thickness of the lighttransmission layer constant on the surface of the substrate. Thus, it isdifficult to stably reproduce the information signal.

Alternatively, the light transmission layer may be formed by adhering athermoplastic resin sheet having a film thickness of 0.1 mm to thesurface of the substrate by the roller pressing method. However, in thismethod, when the sheet is pressed, the sheet deforms and the adhesiveagent protrudes to the reading surface side. Thus, it is difficult toform the light transmission layer with an equal film thickness. It ismuch difficult to stably reproduce the information signal.

Thus, to solve those problems, the inventors of the present inventionhave thought of a method for forming a light transmission layer of whicha sheet composed of an adhesive layer and a light transmission layer isadhered to an information signal portion of a substrate. As a result, amethod for adhering a light transmission layer to a substrate using anadhering device composed of an elastic pad and a metal plane stage hasbeen proposed. Next, with reference to an accompanying drawing, theadhering device will be practically described.

In other words, as shown in FIG. 1, in a conventional adhering device, afixed stage 101 and a movable stage 102 are disposed at their oppositepositions.

The fixed stage 101 is used to hold a sheet 103. Thus, the fixed stage101 is structured so that it can hold the sheet 103. In other words, avertically movable pin 105 that is protruded from the fixed stage 101and buried therein is disposed in the fixed stage 101. The verticallymovable pin 105 faces the movable stage 102. The diameter of thevertically movable pin 105 is the same as the diameter of a through-hole103 a of the sheet 103. The through-hole 103 a of the sheet 103 is fitto the vertically movable pin 105. As a result, the sheet 103 can beheld on the fixed stage 101. A substrate alignment pin 106 cylindricallyprotrudes at an upper portion of the vertically movable pin 105. Thediameter of the substrate alignment pin 106 is almost the same as thediameter of a center hole 104 a of a disc substrate 104. While thesubstrate alignment pin 106 aligns the center of the disc substrate 104,the vertically movable pin 105 holds the disc substrate 104. On thefixed stage 101, the sheet 103 is fit to the substrate alignment pin 106so that the sheet 103 is hold by a shoulder portion of the verticallymovable pin 105.

A pad 107 made of an elastic member such as rubber is disposed on aninner plane of the movable stage 102 so that the pad 107 faces the fixedstage 101. The pad 107 is formed in a conical shape. The plane side ofthe conical shape is secured to the inner plane of the movable stage102.

When the disc substrate 104 and the sheet 103 are adhered by such anadhering device, the through-hole 103 a of the sheet 103 is fit to thevertically movable pin 105 disposed so that the sheet 103 is held on thefixed stage 101. At that point, an adhesive plane 103 b of the sheet 103is disposed so that it faces the movable stage 102. Thereafter, thecenter hole 104 a of the disc substrate 104 is fit to the substratealignment pin 106 so that the center hole 104 a is held on the shoulderportion of the vertically movable pin 105. At that point, the discsubstrate 104 is held on the vertically movable pin 105 so that arecording plane 104 b that has an information signal portion faces theadhesive plane 103 b that has an adhesive layer.

Thereafter, the movable stage 102 is moved toward the fixed stage 101(downwards in FIG. 1). With the pad 107, the substrate alignment pin 106is pressed. Thereafter, the vertically movable pin 105 is moved in thefixed stage 101 through the disc substrate 104. As a result, theclearance between the disc substrate 104 and the sheet 103 graduallybecomes small. Finally, the disc substrate 104 and the sheet 103 arepress-fit. As a result, the recording plane 104 b and the adhesive plane103 b are adhered. After they have been stably press-fit, the movablestage 102 is moved in the direction that it is apart from the fixedstage 101. Thereafter, with a predetermined conveying device (notshown), the disc substrate 104 and the sheet 103, which have beenpress-fit, are removed from the fixed stage 101.

As a result, the disc substrate 104 and the sheet 103 have been adhered.An optical disc of which a light transmission layer has been formed onthe recording plane 104 b of the disc substrate 104 has been produced.

An optical disc having a light transmission layer formed in such amanner can conform with a large NA of the objective lens, which is usedto reproduce data from the optical disc.

However, various experiments and evaluations that the inventors haverepeatedly conducted show that such an optical disc has the followingproblem.

In other words, when a phase change type recording material is used foran information signal portion in an optical disc on which a lighttransmission layer is formed, as the material of the outermost layer ofthe information signal portion, a mixture (ZnS—SiO₂) of zinc sulfide andsilicon oxide, which is a transparent dielectric, are normally used. Alight transmissivity sheet is adhered onto the information signalportion having the ZnS—SiO₂ layer as the outermost layer through anadhesive layer made of a pressure-sensitive adhesive agent is disposed.

However, the results of which the inventors of the present inventionhave conducted acceleration tests for a plurality of optical discshaving such a light transmission layer and measured reflectance thereofshow that the reflectance thereof deteriorates. When the reflectancedeteriorates, it will become difficult to record and reproduce aninformation signal to and from an optical disc having a lighttransmission layer with high accuracy.

Therefore, an object of the present invention is to provide an opticaldisc of which a light transmissivity sheet has been adhered on one mainplane of a substrate through an adhesive layer, in particular, anoptical disc that suppresses the variation of reflectance thereof, thatsuppresses the variation of reflectance on the recording/reproducingplane thereof, that conforms with a large NA of an objective lens usedto record/reproduce data, and that has a light transmission layer thathas small double refraction, high transparency, and equal film thicknessand an optical disc producing method that allows the production yield toimprove.

DISCLOSURE OF THE INVENTION

The inventors of the present invention have intensively made efforts tosolve the aforementioned problems of the prior art. The outline will bedescribed in the following.

As described above, according to the knowledge that the inventors of thepresent invention have obtained, when an optical disc is produced byadhering a light transmissivity sheet and a disc substrate through anadhesive layer made of a pressure-sensitive adhesive agent, thereflectance varies in each of a plurality of optical discs. In addition,the reflectance varies on the recording/reproducing plane. Furtherobservations and experiments that the inventors of the present inventionhave conducted for those optical discs show that the light transmissionlayer has yellowish discolored, which causes the reflectance to vary.

The inventors of the present invention have further pursued causes ofthe yellowish discoloring of the light transmission layer and evaluatedthem. As a result, the inventors of the present invention have supposedthat a chemical reaction of a dielectric layer, which is made ofZnS—SiO₂ used for a laminate film that composes the information signalportion of the optical disc, and the pressure sensitive adhesive agentcauses the light transmission layer to yellowish discolor. Inconsideration of the knowledge of the inventors of the present inventionalong with the supposed cause, it seems that zinc (Zn) mainly affectsthe pressure-sensitive adhesive agent is high and so does sulfur (S).

In the information signal portion of the optical disc, however, to havesufficient recording characteristic and so, as the material of adielectric film that sandwiches the recording layer, ZnS—SiO₂ ispreferably used. When the dielectric film is made of other than ZnS—SiO₂as the material of the dielectric film, which sandwiches the recordinglayer, there is a possibility that a chemical reaction of thepressure-sensitive adhesive agent and the adhesive layer might cause theadhesive layer to discolor. In addition, the material of the dielectricfilm is an important structural element in various characteristics suchas a recording/reproducing characteristic of the optical disc. Thus,since the material of the dielectric film has many restrictions toimprove characteristics, it is very difficult to replace the material ofthe dielectric film with another one.

The inventors of the present invention have repeatedly evaluated amethod for preventing the adhesive layer of the light transmission layerfrom discoloring, in particular, yellowish discoloring and thought of amethod of which a material that does not chemically react with thepressure-sensitive adhesive agent, which composes the adhesive layer, isinterposed between the information signal portion and thepressure-sensitive adhesive agent. In addition, the inventors of thepresent invention have evaluated and experiments various materials thatdo not chemically react with the pressure-sensitive adhesive agent andhas a knowledge that as such a material inorganic materials such asdielectrics for example silicon nitride and silicon oxide and organicmaterials such as ultraviolet ray setting resin are suitable.

According to the evaluated result that the inventors of the presentinvention have intensively conducted, to accomplish good reflectance, itis preferred that on the upper layer of the conventional informationsignal portion of the optical disc, on a lower plane of the adhesivelayer of the light transmission layer, a reaction protection layer madeof a dielectric inorganic material or an organic material should bedisposed. As the material, silicon nitride, silicon oxide, or anultraviolet ray setting resin is more suitable.

To accomplish the aforementioned object, a first aspect of the presentinvention is an optical disc having an information signal portioncomposed of a plurality of layers and configured to record and/orreproduce an information signal and a light transmission layerconfigured to transmit laser light used to record and/or reproduce theinformation signal, the information signal portion and the lighttransmission layer being formed on one main plane of a substrate of theoptical disc,

-   -   wherein the light transmission layer comprises at least a sheet        having light transmissivity and an adhesive layer for adhering        the sheet to the main plane of the substrate, and    -   wherein a reaction protection layer is formed on the information        signal portion so that the reaction protection layer faces the        adhesive layer.

According to the first aspect of the present invention, the reactionprotection layer is typically made of a dielectric. The dielectric ispreferably silicon nitride (SiN, Si₃N₄). The film thickness of thereaction protection layer made of silicon nitride is preferably 2 nm orlarger. The dielectric that composes the reaction protection layer maybe silicon oxide (SiO₂). The film thickness of the reaction protectionlayer made of silicon oxide is preferably 2 nm or larger.

According to the first aspect of the present invention, the filmthickness of the light transmission layer is typically in the range from90 μm to 110 μm.

A second aspect of the present invention is an optical disc having aninformation signal portion composed of a plurality of layers andconfigured to record and/or reproduce an information signal and a lighttransmission layer configured to transmit laser light used to recordand/or reproduce the information signal, the information signal portionand the light transmission layer being formed on one main plane of asubstrate of the optical disc,

wherein the light transmission layer comprises a sheet having lighttransmissivity and an adhesive layer configured to adhere the sheet tothe main plane of the substrate, and

wherein a reaction protection layer made of an organic material isformed between the information signal portion and the adhesive layer.

According to the second aspect of the present invention, the organicmaterial is typically an ultraviolet ray setting resin. The ultravioletray setting resin preferably contains a solvent. The solvent ispreferably methoxypropanol. Alternatively, another solvent may be used.

According to the second aspect of the present invention, thedistribution of the film thickness of the reaction protection layer in aregion of at least the information signal portion is typically 1 μm orsmaller.

According to the second aspect of the present invention, the filmthickness of the light transmission layer is typically in the range from90 μm to 110 μm.

According to the second aspect of the present invention, typically, theorganic material is an ultraviolet ray setting resin, which is hardenedby the radiation of an ultraviolet ray. In reality, the organic materialis an ultraviolet ray hardening resin of for example acrylate group,thiol group, epoxy group, or a silicon group. When an ultraviolet raysetting resin is used as the reaction protection layer, typically, anultraviolet ray is radiated to at least the ultraviolet ray settingresin and then hardened. As a result, the reaction protection layer isformed. According to the present invention, a suitable hardening methodcan be selected for a resin selected as an organic material.

A third aspect of the present invention is an optical disc having aninformation signal portion configured to record and/or reproduce aninformation signal and a light transmission layer configured to transmitlaser light used to record and/or reproduce the information signal, theinformation signal portion and the light transmission layer being formedon one main plane of a substrate of the optical disc,

wherein the light transmission layer comprises a sheet having lighttransmissivity and an adhesive layer configured to adhere the sheet tothe main plane of the substrate,

wherein the information signal portion comprises a reflection layerconfigured to reflect the laser light, a first dielectric layer, arecording layer configured to record the information signal, and asecond dielectric layer successively formed from the substrate, and

wherein the film thickness of the second dielectric layer is designatedso that the reflectance of the laser light on a flat plane of thesubstrate is 15% or larger.

According to the present invention, to allow the reflectance of laserlight to be 15% or larger, the film thickness of the second dielectriclayer is typically in the range from 45 nm to 90 nm or in the range from130 nm to 175 nm.

According to the third aspect of the present invention, the seconddielectric layer is typically made of a mixture of zinc sulfide andsilicon oxide.

A fourth aspect of the present invention is an optical disc producingmethod, comprising the steps of:

forming an information signal portion configured to record and/orreproduce an information signal on a main plane of a substrate; and

adhering a light transmissivity sheet configured to transmit laser lightused to record and/or reproduce the information signal in a region thatcovers the information signal portion through an adhesive layer,

wherein a reaction protection layer is formed on the outermost layer ofthe information signal portion.

According to the fourth aspect of the present invention, the reactionprotection layer is typically made of a dielectric. In reality, thedielectric is silicon nitride or silicon nitride. The film thickness ofthe reaction protection layer made of silicon nitride or silicon oxideis 2 nm or larger.

According to the fourth aspect of the present invention, the adhesivelayer is typically made of a pressure-sensitive adhesive agent.

According to the fourth aspect of the present invention, the filmthickness of the light transmission layer is preferably in the rangefrom 90 μm to 110 μm.

A fifth aspect of the present invention is an optical disc producingmethod, comprising the steps of:

forming an information signal portion configured to record and/orreproduce an information signal on a main plane of a substrate; and

adhering a light transmissivity sheet configured to transmit laser lightused to record and/or reproduce the information signal in a region thatcovers the information signal portion through an adhesive layer,

wherein after the information signal portion forming step, before thelight transmissivity sheet adhering step, a reaction protection layermade of an organic material is formed on at least an upper layer of theinformation signal portion.

According to the fifth aspect of the present invention, to coat anorganic material such as an ultraviolet ray setting resin on all theplane of at least the information signal portion, it is preferred thatafter liquid ultraviolet ray setting resin is dripped on the substrate,the substrate is rotated about the center axis of the disc plane. Inother words, the ultraviolet ray setting resin is formed so that itcoats the information signal portion by for example the spin coatmethod. When the ultraviolet ray setting resin contains a solvent, afterit is coated by the spin coat method, the solvent is evaporated.

According to the fifth aspect of the present invention, the organicmaterial is typically an ultraviolet ray setting resin. To decrease thedifference between the inner and outer circumferential film thicknessesof the substrate, the ultraviolet ray setting resin contains a solvent.According to the fifth aspect of the present invention, the distributionof the film thickness of the reaction protection layer in a region forat least the information signal portion is preferably 1 μm or smaller.

According to the fifth aspect of the present invention, the filmthickness of the light transmission layer is typically in the range from90 μm to 110 μm.

According to the present invention, the film thickness of theinformation signal portion is preferably in the range from 183 nm to 313nm.

According to the present invention, to suppress the chemical reaction ofthe adhesive layer and the outermost layer of the information signallayer and prevent the adhesive layer from discoloring, the reactionprotection layer is typically made of a dielectric. To securely suppressthe discoloring of the adhesive layer, the dielectric is preferablysilicon nitride (Si₃N_(4−x) (where 0×1, typically Si₃N₄) or siliconoxide (SiO_(x) (where 1×2, typically SiO₂ or SiO). The dielectric may bealuminum nitride (AlN_(x) (where 0.5×1, typically AlN), aluminum oxide(Al₂O_(3−x) (where 0×1, typically alumina (Al₂O₃)), magnesium oxide(MgO), yttrium oxide (Y₂O₃), magnesium aluminum oxide (MgAl₂O₄),titanium oxide (TiOx (where 1×2, typically TiO₂), barium titanium oxide(BaTiO₃), strontium titanium oxide (SrTiO₃), tantalum oxide (Ta₂O_(5−x)(where 0×1, typically Ta₂O₅), germanium oxide (GeO_(x)) (where 1×2),silicon carbide (SiC), zinc sulfide (ZnS), lead sulfide (PbS), Ge—N,Ge—N—O, Si—N—O, calcium fluoride (CaF), lanthanum fluoride (LaF),magnesium fluoride (MgF₂), sodium fluoride (NaF), titanium fluoride(TiF₄), or the like. Moreover, a material whose principal component isone of those materials or a mixture of these materials for exampleAlN—SiO₂ may be used.

According to the present invention, the substrate typically has a discshape. In addition, the light transmissivity sheet has a disc shape.According to the present invention, to form a light transmission layerhaving a light transmissivity sheet, an adhesive layer is disposed onone plane of a sheet that has been cut in a disc shape. By adhering thesubstrate and the sheet through the adhesive layer, a light transmissionlayer is formed on the substrate. According to the present invention,typically, the inner diameter of the disc shaped sheet is equal to orlarger than the inner diameter of the disc shaped substrate. Inaddition, the outer diameter of the disc shaped sheet is equal to orsmaller than the disc shaped substrate.

According to the present invention, the adhesive layer is typically madeof a Pressure-Sensitive Adhesive agent (PSA).

According to the present invention, to minimize the warp and distortionof the optical disc to be produced, the light transmissivity sheet ispreferably made of the same material as the substrate. The thickness ofthe light transmissivity sheet is typically smaller than the thicknessof the substrate. In reality, the thickness of the light transmissivitysheet is in the range from 30 μm to 150 μm. In addition, according tothe present invention, the disc substrate is made of a resin having alow moisture absorbing power such as polycarbonate (PC) or cycloolefinpolymer. The light transmissivity sheet is preferably made of the samematerial as the disc substrate. The substrate may be for example asubstrate made of a metal such as aluminum, a glass substrate, or asubstrate made of polyolefin, polyimide, polyamide, polyphenylenesulfide, polyethylene terephthalate (PET), or the like. The lighttransmissivity sheet is typically made of polycarbonate resin.Alternatively, the light transmissivity sheet may be made of anothermaterial.

According to the present invention, to prevent a foreign matter presenton the sheet holding plane of the adhering device from flawing ordenting the light transmissivity sheet, the sheet is preferably composedof a light transmissivity sheet, an adhesive layer formed on one placeof the light transmissivity sheet, and a protection layer formed on theother plane of the light transmissivity sheet. In addition, theprotection layer is preferably made of a polyethylene terephthalate(PET) sheet, a polyethylene naphthalate (PEN) sheet, or the like. Morepractically, a second adhesive agent is coated on at least one plane ofthe PET sheet or the PEN sheet. The plane on which the second adhesiveagent has been coated is adhered on one plane of the lighttransmissivity sheet. As a result, a sheet that is adhered to the discsubstrate is obtained.

According to the present invention, the light transmissivity sheet istypically made of a non-magnetic material that can transmit laser lightradiated from a GaN semiconductor laser (wavelength of light emission:400 nm band, blue light emission), a ZnSe semiconductor laser(wavelength of light emission: 500 nm band, green light emission), anAlGaInP semiconductor laser (wavelength of light emission: around 635 to680 nm, red light emission), or the like, which is used torecord/reproduce at least an information signal. In reality, the lighttransmissivity sheet is made of a thermoplastic resin such aspolycarbonate that has a light transmission characteristic.

The present invention can be preferably applied to an optical dischaving a thin light transmission layer. In other words, the presentinvention can be applied to the so-called DVR-red, of which aninformation signal is recorded and reproduced using a semiconductorlaser whose wavelength of light emission is around 650 nm or theso-called DVR-blue, of which an information signal is recorded andreproduced with a semiconductor laser whose wavelength of light emissionis around 400 nm. Preferably, using an objective lens whose NA is ashigh as around 0.85 having two lens elements disposed in series, the DVRrecords an information signal. Practically, one side of the optical dischas a recording capacity of around 22 GB. The optical disc according tothe present invention is an optical disc such as a DVR, which is housedin a cartridge. However, the present invention is not limited to anoptical disc that is housed in a cartridge.

In the optical disc and the producing method thereof according to thepresent invention, a light transmission layer is composed of at least alight transmissivity sheet and an adhesive layer that adheres the lighttransmissivity sheet to a main plane of a substrate. A layer thatcomposes a plane that contacts the adhesive layer of the informationsignal portion is composed of a reaction protection layer. As a result,the adhesive layer and a dielectric layer that composes the informationsignal portion can be prevented from chemically reacting with eachother.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing a conventional adhering devicethat is used to adhere a disc substrate and a sheet;

FIG. 2 is a sectional view showing an optical disc according to a firstembodiment of the present invention;

FIG. 3 is a sectional view showing a disc substrate according to thefirst embodiment of the present invention;

FIG. 4 is a sectional view showing the detail of an information signalportion of the disc substrate according to the first embodiment of thepresent invention;

FIG. 5 is a sectional view showing a sheet used to form a lighttransmission layer according to the first embodiment of the presentinvention;

FIG. 6 is a schematic diagram showing an adhering device that adheresthe disc substrate and the sheet according to the first embodiment ofthe present invention;

FIG. 7 is a graph showing a film thickness dependency of a seconddielectric layer to the reflectance of a mirror portion of the opticaldisc according to the first embodiment of the present invention;

FIG. 8 is a sectional view showing an optical disc according to a thirdembodiment of the present invention;

FIG. 9 is a sectional view showing a disc substrate according to thethird embodiment of the present invention;

FIG. 10 is a schematic diagram for explaining a method for forming areaction protection resin layer according to the third embodiment of thepresent invention;

FIG. 11 is a schematic diagram for explaining the method for forming thereaction protection resin layer according to the third embodiment of thepresent invention; and

FIG. 12 is a schematic diagram for explaining the method for forming thereaction protection resin layer according to the third embodiment of thepresent invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Next, with reference to the accompanying drawings, embodiments of thepresent invention will be described. In the drawings with which theembodiments will be described, similar portions are denoted by similarreference numerals.

First of all, an optical disc according to a first embodiment will bedescribed. FIG. 2 shows the optical disc according to the firstembodiment of the present invention.

As shown in FIG. 2, in the optical disc according to the firstembodiment, a center hole 1 b is formed at a center portion of a replicasubstrate 1 a of a disc substrate 1. One main plane of the discsubstrate 1 is unevenly formed. An information signal portion 1 c isformed on the main plane of the disc substrate 1. In addition, a lighttransmission layer 2 is disposed on the disc substrate 1. The lighttransmission layer 2 is composed of a light transmissivity sheet 2 a andan adhesive layer 2 b that has been adhered thereto. A through-hole 2 cis formed at a center portion of the light transmission layer 2.

A circular clamp region 3 is set around the through-hole 2 c on a lowermain plane of the light transmissivity sheet 2 a of the lighttransmission layer 2. The innermost diameter of the circular clampregion 3 is for example 23 mm. The outermost diameter of the circularclamp region 3 is for example 33 mm. On the lower main plane of thelight transmissivity sheet 2 a of the light transmission layer 2 in thecircular clamp region 3, a clamp reference plane 3 a at which theoptical disc is clamped or held by a spindle of a recording andreproducing device (both are not shown) is set. Since the lighttransmissivity sheet 2 a has been adhered to the lower main plane of thedisc substrate 1 through the adhesive layer 2 b, the diameter of thethrough-hole 2 c is designated equal to or larger than the diameter ofthe center hole 1 b of the disc substrate 1. For example, the diameterof the through-hole 2 c is designated 15 mm or larger. In addition,since the clamp reference plane 3 a is set on the lower main plane ofthe light transmissivity sheet 2 a of the light transmission layer 2,the diameter of the through-hole 2 c is equal to or smaller than theinnermost diameter of the circular clamp region 3. In reality, thediameter of the through-hole 2 c is for example 23 mm or smaller.

Next, a method for producing the optical disc according to the firstembodiment will be described. FIG. 3 shows a disc substrate that is aholding member of the optical disc. FIG. 4 shows the detail of the discsubstrate. FIG. 5 shows a sheet adhered to the disc substrate.

In the optical disc producing method according to the first embodiment,as shown in FIG. 3, a disc substrate 1 on which a light transmissionlayer is formed is prepared. In the disc substrate 1, a center hole 1 bis formed at a center portion of a replica substrate 1 a. An informationsignal portion 1 c is formed on a lower main plane of the disc substrate1. The lower main plane is unevenly formed.

The replica substrate 1 a is produced by the injection molding methodusing a predetermined stamper. The thickness of the replica substrate 1a is in the range from for example 0.6 to 1.2 mm. As the material of thereplica substrate 1 a, for example a resin that has a low moistureabsorption property such as polycarbonate or cycloolefin polymer (forexample, ZEONEX, which is a trademark of Nihon Zeon) is used. Thereplica substrate 1 a may be for example a substrate made of a metalsuch as aluminum, a glass substrate, or a substrate made of polyolefin,polyimide, polyamide, polyphenylene sulfide, polyethylene terephthalate,or the like. In addition, on the uneven portion formed on the lower mainplane of the replica substrate 1 a, a recording film, a reflection film,and so forth are formed. Those films compose the information signalportion 1 c. The information signal portion 1 c is composed of areflection film, a film made of a magneto-optical material, a film madeof a phase change material, or an organic color matter film. Amongthose, as the material of the reflection film, for example an Al alloyis used. In reality, when the optical disc as the final product is aread-only memory (ROM) optical disc, the information signal portion 1 cis composed of a single layer film or a laminate film that has at leasta reflection layer made of for example an Al alloy. When the opticaldisc as the final product is a rewritable optical disc, the informationsignal portion 1 c is composed of a single layer film or a laminate filmthat has at least a film made of a magneto-optical material or a filmmade of a phase-change material. When the optical disc as the finalproduct is a write-once type optical disc, the information signalportion 1 c is composed of a single layer film or a laminate film thathas at least a film made of an organic color matter material.

As shown in FIG. 4, in the disc substrate 1 according to the firstembodiment, a polycarbonate (PC) substrate that has a thickness of forexample 1.1 mm and that has a disc shape is used as the replicasubstrate 1 a. The diameter (outer diameter) of the replica substrate 1a is for example 120 mm. The aperture (inner diameter) of the centerhole 1 b is for example 15 mm. On a reflection layer 11, a firstdielectric layer 12, a recording layer 13, a second dielectric layer 14,and a reaction protection layer 15 are successively formed. Those layerscompose the information signal portion 1 c.

As shown in FIG. 5, a sheet 4 as the light transmission layer 2 iscomposed of a light transmissivity sheet 2 a and an adhesive layer 2 bmade of a pressure-sensitive adhesive agent (PSA) coated on one plane ofthe light transmissivity sheet 2 a. As with the disc substrate 1, thesheet 4 is cut in a disc shape. At a center portion of the sheet 4, athrough-hole 2 c is formed. The diameter (outer diameter) of the sheet 4is designated almost the same as or smaller than the outer diameter ofthe disc substrate 1. In reality, the diameter of the sheet 4 is forexample 120 mm. The diameter (inner diameter) of the through-hole 2 c isdesignated equal to or larger than the aperture of the center hole 1 b.The innermost diameter of the circular clamp region 3 is designatedequal to or smaller than the innermost diameter of the circular clampregion 3 (for example, 23 mm). The diameter of the through-hole 2 c isfor example 23 mm.

The light transmissivity sheet 2 a of the sheet 4 is made of athermoplastic resin that has light transmissivity that satisfies atleast an optical characteristic that can transmit laser light used forrecording/reproducing. The material of the thermoplastic resin is closeto the material of the replica substrate 1 a in heat resistancedimension stability, coefficient of thermal expansion, or coefficient ofmoisture absorption expansion. In reality, the thermal plastic resin isa methacrylic resin such as polycarbonate (PC) or polymethylmethacrylate. The thickness of the light transmissivity sheet 2 a ispreferably designated in the range from 50 to 100 μm, more preferably inthe range from 60 to 80 μm. According to the first embodiment, since thelight transmissivity sheet 2 a is adhered to one main plane of the discsubstrate 1 through the adhesive layer 2 b made of a pressure-sensitiveadhesive agent (PSA), the thickness of the light transmissivity sheet 2a is designated for example 70 μm. The thickness of the lighttransmissivity sheet 2 a is designated depending on the wavelength oflaser light used for recording/reproducing an information signal and adesired film thickness of the light transmission layer 2.

The PSA, which composes the adhesive layer 2 b, is for example acrylicresin. The thickness of the adhesive layer 2 b is for example 30 μm.However, the thickness of the adhesive layer 2 b and the material of thepressure-sensitive adhesive agent are designated depending on thedesired film thickness of the light transmission layer 2 and thewavelength of the laser light used for recording/reproducing theinformation signal. While the sheet 4 is stocked, a protection film islaminated on the adhesive layer 2 b of the sheet 4.

Next, a method for adhering the disc substrate 1 and the sheet 4 will bedescribed.

First of all, an adhering device used to adhere the disc substrate 1 andthe sheet 4 will be described. FIG. 6 shows the adhering deviceaccording to the first embodiment.

As shown in FIG. 6, in the adhering device according to the firstembodiment, a fixed stage 21 and a movable stage 22 are disposed attheir opposite positions.

The fixed stage 21 is structured so that it can hold a sheet 4. In otherwords, in the fixed stage 21, a vertically movable pin 23 that isprotruded from the fixed stage 21 and buried therein is disposed at aportion that faces the movable stage 22. The diameter of the verticallymovable pin 23 is the same as the diameter of the through-hole 2 c ofthe sheet 4. The through-hole 2 c of the sheet 4 is fit to thevertically movable pin 23. The sheet 4 is held on the fixed stage 21. Asubstrate alignment pin 24 that protrudes in a cylinder shape isdisposed at an upper portion of the vertically movable pin 23. Thediameter of the substrate alignment pin 24 is almost the same as thediameter of the center hole 1 b of the disc substrate 1. While thesubstrate alignment pin 24 is aligned with the center of the discsubstrate 1, the disc substrate 1 is held with the vertically movablepin 23. On the fixed stage, the sheet 4 is fit to the vertically movablepin 23 so that the sheet 4 is held. While the disc substrate 1 is fit tothe substrate alignment pin 24, the disc substrate 1 is held by ashoulder portion of the vertically movable pin 23.

On a lower plane of the movable stage 22, a pad 25 made of an elasticmember for example rubber is disposed so that the pad 25 faces the fixedstage 21. The pad 25 has a circular cone shape. The plane side of thecircular cone shape of the pad 25 is secured to the lower plane of themovable stage 22.

When the disc substrate 1 and the sheet 4 are adhered by the adheringdevice according to the first embodiment, the through-hole 2 c of thesheet 4 is fit to the vertically movable pin 23 so that the sheet 4 isheld on the fixed stage 21. At that point, the sheet 4 is placed so thatthe adhesive layer 2 b faces the movable stage 22. Thereafter, thecenter hole 1 b of the disc substrate 1 is fit to the substratealignment pin 24 so that the disc substrate 1 is held on the shoulderportion of the vertically movable pin 23. At that point, the discsubstrate 1 is held by the vertically movable pin 23 so that theinformation signal portion 1 c that has the information signal portionfaces the adhering plane of the adhesive layer 2 b.

Thereafter, the movable stage 22 is moved toward the fixed stage 21(downwards in FIG. 6). With the pad 25, the substrate alignment pin 24is pressed. Thereafter, the vertically movable pin 23 is moved in thefixed stage 21. As a result, the clearance between the disc substrate 1and the sheet 4 is gradually decreased. Finally, the disc substrate 1and the sheet 4 are press-fit. As a result, the plane of the informationsignal portion 1 c and the adhesive layer 2 b are adhered. After theyhave been stably press-fit, the movable stage 22 is moved so that it isapart from the fixed stage 21. Thereafter, with a predeterminedconveying device (not shown), the disc substrate 1 and the sheet 4 whichhave been press-fit are removed from the fixed stage 21.

As a result, the disc substrate 1 and the sheet 4 has been adhered. Anoptical disc of which the light transmission layer 2 has been formed onthe plane of the information signal portion 1 c of the disc substrate 1has been produced.

The inventors of the present invention have produced various types ofdisc substrates 1 in which the film thickness of the reaction protectionlayer 15 had been varied and have compared them. In reality, as thereplica substrate 1 a, a polycarbonate (PC) substrate that has athickness of 1.1 mm and has a disc shape is used. As the informationsignal portion 1 c, a laminate film of a reflection layer 11 made of anAl alloy that has a film thickness of 100 nm, a first dielectric layer12 that has a film thickness of 18 nm and that is made of a mixture ofZnS and SiO₂ (ZnS—SiO₂), a recording layer 13 that has a film thicknessof 24 nm and that is made of a GeInSbTe alloy layer, a second dielectriclayer 14 that is made of ZnS—SiO₂, and a reaction protection layer 15that is made of a dielectric is used.

The film thickness of the second dielectric layer 14, made of ZnS—SiO₂,of the information signal portion 1 c is designated so that thereflectance of the mirror portion becomes 15% or larger. In other words,in grooves used to record/reproduce an information signal on the opticaldisc, the reflectance of 10% or larger is required. The reflectance ofthe grooves is around 70% of the reflectance of the flat mirror portion.Thus, to obtain good recording/reproducing characteristics on thegrooves, the reflectance of the mirror portion should be 15% or larger.In such a consideration, the inventors of the present invention havemeasured the film thickness dependency of the second dielectric layer(ZnS—SiO₂) to the reflectance.

FIG. 7 shows the measured results. As shown in FIG. 7, the inventors ofthe present invention have thought of the fact that as the filmthickness of the second dielectric layer 14 increases, the reflectancevaries in a sine wave shape. Thus, the film thickness of the seconddielectric layer 14 is designated so that the reflectance, which variesin a sine wave shape, becomes 15% or larger. In reality, the filmthickness of the second dielectric layer 14 is designated in the rangefrom 45 to 90 nm or in the range from 130 to 175 nm. According to thefirst embodiment, the film thickness of the second dielectric layer 14is designated 140 nm.

The material of the reaction protection layer 15 of the informationsignal portion 1 c is decided in the following manner. In other words,as described above, the light transmissivity sheet 2 a is adhered on theinformation signal portion 1 c through the adhesive layer 2 b made of apressure-sensitive adhesive agent (PSA). Thus, as the material of thereaction protection layer 15, which composes the outermost layer of theinformation signal portion 1 c, a material that does not chemicallyreact with the adhesive layer 2 b is selected. In reality, for example,silicon nitride is selected. According to the first embodiment, thereaction protection layer 15 is made of Si₃N₄.

A disc substrate 1 according to the first embodiment is a disc substrateof which the outermost layer of the information signal portion 1 c isthe reaction protection layer 15 made of Si₃N₄. A disc substrate 1 ofwhich the film thickness of the reaction protection layer 15 is 1 nm isreferred to as disc substrate 1 of Example 1. A disc substrate 1 ofwhich the film thickness of the reaction protection layer 15 is 2 nm isreferred to as disc substrate 1 of Example 2. A disc substrate 1 ofwhich the film thickness of the reaction protection layer 15 is 5 nm isreferred to as disc substrate 1 of Example 3. A disc substrate 1 ofwhich the film thickness of the reaction protection layer 15 is 10 nm isreferred to as disc substrate 1 of Example 4. A disc substrate 1 ofwhich the film thickness of the reaction protection layer 15 is 30 nm isreferred to as disc substrate 1 of Example 5.

A disc substrate 1 of which a disc shaped PC substrate that has athickness of 1.1 mm is used as the replica substrate 1 a, the diameter(outer diameter) of the PC substrate is for example 120 mm, and theaperture (inner diameter) of the center hole 1 b is for example 15 mm isreferred to as disc substrate 1 of Comparative Example. The informationsignal portion 1 c of the conventional disc substrate 1 is composed of alaminate of a reflection layer made of an Al alloy having a filmthickness of 100 nm, a first dielectric layer made of ZnS—SiO₂ having afilm thickness of 18 nm, a phase change recording layer made of aGeInSbTe alloy having a film thickness of 24 nm, and a second dielectriclayer made of ZnS—SiO₂ having a film thickness of 140 nm. In otherwords, the disc substrate that does not have a reaction protection layer15 is referred to as disc substrate 1 of Comparative Example.

The rates of variation of reflectance of the optical discs produced bythe adhering method using the disc substrates 1 of Example 1 to Example5 and Comparative Example have been measured. First of all, the initialcrystallizing process has performed for the optical disc produced withthe disc substrates of Example 1 to Example 5 and Comparative Example.As a result, the recording layer 13 of the information signal portion 1c has been changed from the amorphous state to the crystal state.Thereafter, the initial reflectance has been measured. The optical discshave been placed in a constant-temperature, constant-humidity tank inwhich temperature has been kept at 80° C. and the humidity has been keptat 85% for 1000 hours. In those conditions, acceleration tests have beenperformed. Thereafter, the reflectance of the optical discs taken fromthe constant-temperature, constant-humidity tank has been measured.Thereafter, the variation of reflectance before and after theacceleration tests has been measured. The reflectance has been measuredfor a non-uneven portion of the optical discs namely mirror portionthereof using an evaluating system for an optical system having awavelength of 400 nm and a numerical aperture NA of 0.85. The rate ofvariation of reflectance has been obtained by dividing the amount ofvariation of reflectance of which the reflectance after the accelerationtests is subtracted from the initial reflectance by the initialreflectance. Table 1 shows the measured results of the rates ofvariation of reflectance.

TABLE 1 Comparative example Example 1 Example 2 Example 3 Example 4Example 5 Film thickness of Si₃N₄ 0 1 2 5 10 30 [nm] Initial reflectance[%] 20.0 20.1 20.7 21.2 21.6 18.0 Reflectance after 14.3 17.2 18.8 19.920.8 17.8 acceleration test [%] Amount of variation of 5.7 2.9 1.9 1.30.8 0.2 reflectance [%] Amount of variation of 0.285 0.144 0.092 0.0610.037 0.011 reflectance/ Initail reflectance

Table 1 shows that the amounts of variation of reflectance (rates ofvariation of reflectance) to the initial reflectance of the accelerationtests for the optical discs produced with the disc substrates 1 ofExample 1 to Example 5 (namely, optical discs of Example 1 to Example 5)are in the range from 0.011 to 0.144 and that the rate of variation ofreflectance to the initial reflectance of the acceleration test for theoptical disc produced with the disc substrate 1 of Comparative Example(namely, optical disc of Comparative Example) is 0.285, which is aroundtwice as large as that of Example 1. In addition, yellowish discoloringhas been observed in the adhesive layer 2 b of the optical disc ofComparative Example. The knowledge that the inventors of the presentinvention have obtained through the experiments represents that theyellowish discoloring of the adhesive layer 2 b is a cause of thedecrease of the reflectance. Thus, with the reaction protection layer 15disposed at the interface with the adhesive layer 2 b of the informationsignal portion 1 c, the reflectance can be prevented from deteriorating.

Table 1 also shows that in the optical discs of Example 1 to Example 5,since the rate of variation of reflectance of the optical disc ofExample 1, where the film thickness of the reaction protection layer 15made of Si₃N₄ is 1 nm, is 0.144 and the rate of variation of reflectanceof the optical disc of example 2, where the film thickness of thereaction protection layer 15 is 2 nm, is 0.092, it is clear that therate of variation of reflectance of the optical disc of Example 2decreases against that of the optical disc of Example 1. In other words,it is clear that the deterioration of the reflectance of the opticaldisc of Example 2 is suppressed against the optical disc of Example 1.When other examples are compared in the same manner, it is clear that asthe film thickness of the reaction protection layer 15 is increased, therate of variation of reflectance is decreased as with the optical discof Example 3 to the optical disc of Example 2; the optical disc ofExample 4 to the optical disc of Example 3; and the optical disc ofExample 5 to the optical disc of Example 4. In addition, the resultsthat the inventors of the present invention have observed the opticaldiscs of Example 1 to Example 5 represent that with the reactionprotection layer 15, the yellowing discoloring is decreased.

When the rate of variation of reflectance exceeds 0.1, therecording/reproducing characteristics of an information signal tend todeteriorate. Thus, the film thickness of the reaction protection layer15 made of Si₃N₄ should be larger than 1 nm, preferably 2 nm or larger.When the film thickness of the reaction protection layer 15 is 2 nm orlarger, it is certain that the rate of variation of reflectance is 0.1or smaller. As a result, good recording/reproducing characteristics areacquired.

As described above, in the optical disc and the producing method thereofaccording to the first embodiment, since the reaction protection layer15 made of Si₃N₄, which does not chemically react with the adhesivelayer 2 b, is disposed as the outermost layer of the information signalportion 1 c on the lower main plane of the disc substrate 1, when theoptical disc is used for a long time, the adhesive layer 2 b can beprevented from yellowish discoloring at the interface with theinformation signal portion 1 c. As a result, the decrease of thereflectance and the deterioration of the optical disc due to thediscoloring can be suppressed. Since the optical disc according to thefirst embodiment of the present invention has the light transmissionlayer, which is thin and which has small double refraction, hightransparency, and equal thickness, the optical disc conforms with alarge NA of an objective lens and has good recording/reproducingcharacteristics.

Next, an optical disc according to a second embodiment of the presentinvention will be described. Unlike with the optical disc according tothe first embodiment, in the optical disc according to the secondembodiment, the reaction protection layer 15 is made of silicon oxide(SiO₂).

The material of the reaction protection layer 15 is decided in thefollowing manner. As described above, a light transmissivity sheet 2 ais adhered to the outermost layer of an information signal portion 1 cthrough an adhesive layer 2 b made of a pressure-sensitive adhesiveagent (PSA). Thus, as the material of the reaction protection layer 15,which composes the outermost layer of the information signal portion 1c, a material that does not chemically react with the adhesive layer 2b, practically, for example silicon oxide, is selected. According to thesecond embodiment, the reaction protection layer 15 is made of SiO₂.

A disc substrate 1 according to the second embodiment, where theoutermost layer of the information signal portion 1 c is the reactionprotection layer 15 made of SiO₂, is hereinafter referred to as discsubstrate 1 of each of examples. A disc substrate 1 of which the filmthickness of the reaction protection layer 15 is 1 nm is referred to asdisc substrate 1 of Example 6. A disc substrate 1 of which the filmthickness of the reaction protection layer 15 is 2 nm is referred to asdisc substrate 1 of Example 7. A disc substrate 1 of which the filmthickness of the reaction protection layer 15 is 5 nm is referred to asdisc substrate 1 of Example 8. A disc substrate 1 of which the filmthickness of the reaction protection layer 15 is 10 nm is referred to asdisc substrate 1 of Example 9. A disc substrate 1 of which the filmthickness of the reaction protection layer 15 is 30 nm is referred to asdisc substrate 1 of Example 10. A disc substrate 1 used to compare thosedisc substrates 1 is the same as that according to the first embodiment.This disc substrate 1 is referred to as disc substrate 1 of ComparativeExample.

Since the other structure of the optical discs according to the secondembodiment is the same as that according to the first embodiment, thedescription will be omitted. In addition, since the optical discproducing method according to the second embodiment is the same as thataccording to the first embodiment, the description will be omitted.

The rates of variation of reflectance of optical discs having the lighttransmission layer formed by the adhering method according to the firstembodiment with the disc substrates 1 of Example 6 to Example 10 andComparative Example have been measured.

In other words, the initial crystallizing process has been performed forthe optical discs produced with the disc substrates 1 of Example 6 toExample 10 and Comparative Example. As a result, the recording layer 13of the information signal portion 1 c has been changed from theamorphous state to the crystal state. Thereafter, the initialreflectance has been measured. The optical discs have been placed in aconstant-temperature, constant-humidity tank in which the temperaturehas been kept at 80° C. and the humidity has been kept at 85% for 1000hours. In those conditions, acceleration tests have been performed.Thereafter, the reflectance of the optical discs taken from theconstant-temperature, constant-humidity tank has been measured.Thereafter, the variation of reflectance before and after theacceleration tests has been measured. The reflectance has been performedat a non-uneven portion namely mirror portion thereof using anevaluating device for an optical system having a wavelength of 400 nmand a numerical aperture NA of 0.85. Table 2 shows the measured resultsof the rates of variations of reflectance.

TABLE 2 Comparative Example example Example 6 Example 7 Example 8Example 9 10 Film thickness of SiO₂ 0 1 2 5 10 30 [nm] Initialreflectance [%] 20.0 20.1 20.6 21.1 21.3 18.4 Reflectance after 14.317.1 18.8 19.7 20.6 17.9 acceleration test [%] Amount of variation of5.7 3.0 1.8 1.4 0.7 0.5 reflectance [%] Amount of variation of 0.2850.149 0.087 0.066 0.033 0.027 reflectance/ Initail reflectance

Table 2 shows that the amounts of variation of reflectance (rates ofvariation of reflectance) to the initial reflectance of the accelerationtests for the optical discs produced with the disc substrates 1 ofExample 6 to Example 10 (namely, optical discs of Example 6 to Example10) are in the range from 0.027 to 0.149 and that the rate of variationof reflectance to the initial reflectance of the acceleration test forthe optical disc produced with the disc substrate 1 of ComparativeExample (namely, optical disc of Comparative Example) is 0.285, which isaround twice as large as that of Example 6. In addition, as described inthe first embodiment, yellowish discoloring has been observed in theadhesive layer 2 b of the optical disc of Comparative Example. Theyellowish discoloring of the adhesive layer 2 b is a cause of thedecrease of the reflectance. Thus, when the reaction protection layer15, which is made of SiO₂ and which does not chemically react with theadhesive layer 2 b, is disposed at the interface with the adhesive layer2 b of the information signal portion 1 c, the reflectance can beprevented from deteriorating.

Table 2 also shows that in the optical discs of Example 6 to Example 10,since the rate of variation of reflectance of the optical disc ofExample 6, where the film thickness of the reaction protection layer 15made of SiO₂ is 1 nm, is 0.149 and the rate of variation of reflectanceof the optical disc of Example 7, where the film thickness of thereaction protection layer 15 is 2 nm, is 0.087, it is clear that therate of variation of reflectance of the optical disc of Example 7decreases against that of the optical disc of Example 6. In other words,it is clear that the deterioration of the reflectance of the opticaldisc of Example 7 is suppressed against the optical disc of Example 6.When other examples are compared in the same manner, it is clear that asthe film thickness of the reaction protection layer 15 is increased, therate of variation of reflectance is decreased as with the optical discof Example 8 to the optical disc of Example 7; the optical disc ofExample 9 to the optical disc of Example 8; and the optical disc ofExample 10 to the optical disc of Example 9. In addition, the resultsthat the inventors of the present invention have observed the opticaldiscs of Example 6 to Example 10 represent that with the reactionprotection layer 15, the yellowing discoloring is decreased.

When the rate of variation of reflectance exceeds 0.1, therecording/reproducing characteristics of an information signal tend todeteriorate. Thus, the film thickness of the reaction protection layer15 made of SiO₂ should be larger than 1 nm, preferably 2 nm or larger.When the film thickness of the reaction protection layer 15 is 2 nm orlarger, it is certain that the rate of variation of reflectance is 0.1or smaller. As a result, good recording/reproducing characteristics areacquired.

In the optical disc and the producing method thereof according to thesecond embodiment, since the reaction protection layer 15 made of SiO₂,which does not chemically react with the adhesive layer 2 b, is disposedas the outermost layer of the information signal portion 1 c on thelower main plane of the disc substrate 1, the same effect as the firstembodiment can be obtained.

Next, an optical disc according to a third embodiment of the presentinvention will be described. FIG. 8 shows the optical disc according tothe third embodiment of the present invention.

As shown in FIG. 8, unlike with the first embodiment, in the opticaldisc according to the third embodiment, a reaction protection resinlayer 31 made of ultraviolet ray setting resin is formed so that itcoats an information signal portion 1 c. A light transmissivity sheet 2a is adhered to the front surface of the reaction protection resin layer31 through an adhesive layer 2 b. In such a manner, an optical disc isstructured.

As shown in FIG. 9, unlike with the information signal portions 1 caccording to the first and second embodiments, the information signalportion 1 c according to the third embodiment does not have a reactionprotection layer 15. In the disc substrate 1 according to the thirdembodiment, a reaction protection resin layer 31 is formed on all onemain plane of a disc substrate 1. A disc substrate 1 that does not havethe reaction protection resin layer 31 is referred to as disc substrate1 of Comparative Example as with the first embodiment.

Next, an optical disc producing method according to the third embodimentwill be described. On an uneven portion formed on one main plane of areplica substrate 1 a that is the same as that according to the firstembodiment, a recording film, a reflection film, and so forth have beenformed. These films compose the information signal portion 1 c.

As shown in FIG. 9, in the disc substrate 1 according to the thirdembodiment, as the replica substrate 1 a, a polycarbonate (PC) substratethat has a thickness of 1.1 mm and that has a disc shape is used. Thediameter (outer diameter) of the PC substrate is for example 120 mm. Theaperture (inner diameter) of a center hole 1 b is for example 15 mm. Theinformation signal portion 1 c is a laminate film of a reflection layer11 made of an Al allow having a film thickness of 100 nm, a firstdielectric layer 12 that has a film thickness of 18 nm and that is amixture of ZnS and SiO₂, a phase change recording layer 13 that has afilm thickness of 24 nm and that is made of a GeSbTe alloy layer, and asecond dielectric layer 14 made of a mixture (ZnS—SiO₂) of zinc sulfide(ZnS) and silicon oxide (SiO₂) that are successively layered. As withthe first embodiment, the film thickness of the second dielectric layer14 made of ZnS—SiO₂ is designated so that the reflectance of a mirrorportion becomes 15% or larger. In reality, the film thickness of thesecond dielectric layer 14 is designated in the range from 45 to 90 nmor in the range from 130 to 175 nm. According to the third embodiment,the film thickness of the second dielectric layer 14 is designated 140nm.

Since the sheet 4 is the same as those according to the first and secondembodiments, the description thereof will be omitted.

Next, a method for forming the reaction protection resin layer 31according to the third embodiment will be described. FIG. 10 to FIG. 12show the method for forming the reaction protection resin layer 31according to the third embodiment.

As shown in FIG. 10, ultraviolet ray setting resin 32 is supplied to andcoated on one main plane of the information signal portion 1 c of thedisc substrate 1. The ultraviolet ray setting resin 32 is supplied froma nozzle opening of an ultraviolet ray setting resin supplying portion33 to the inner circumferential side of the disc substrate 1 so that theultraviolet ray setting resin 32 is formed in for example a disc shape.

Next, as shown in FIG. 11, the disc substrate 1 on which the ultravioletray setting resin 32 has been coated is rotated counterclockwise (in thearrow direction M shown in FIG. 11) about a rotating shaft (not shown)of the device. As a result, the ultraviolet ray setting resin 32 isfully coated on the front surface of the disc substrate 1. Since thedisc substrate 1 is rotated counterclockwise, excessive ultraviolet raysetting resin 32 is shaken off. As a result, the ultraviolet ray settingresin 32 is equally coated on the information signal portion 1 c of thereplica substrate 1 a. Thus, a film having an equal thickness is formed.The film thickness of the ultraviolet ray setting resin 32 that has beenshaken off can be adjusted with the rotation speed.

Next, as shown in FIG. 12, the replica substrate 1 a on which theultraviolet ray setting resin 32 has been coated is placed in aradiation range of an ultraviolet ray light source 34 that can emit anultraviolet ray. At that point, the replica substrate 1 a is placed sothat the coated side of the ultraviolet ray setting resin 32 faces theultraviolet ray light source 34. Thereafter, an ultraviolet ray isradiated from the ultraviolet ray light source 34 to the ultraviolet raysetting resin 32 on the upper main plane of the replica substrate 1 a.At that point, the cumulative intensity of the ultraviolet ray is forexample 500 mJ/cm². With the radiation of the ultraviolet ray, theultraviolet ray setting resin 32, which coats the information signalportion 1 c on the replica substrate 1 a, is hardened. As a result, thereaction protection resin layer 31 has been formed. Thus, the discsubstrate 1 according to the third embodiment has been produced.

The inventors of the present invention have formed the reactionprotection resin layers 31 on the disc substrates 1 produced by theforgoing producing method in various production conditions. A discsubstrate on which the reaction protection resin layer 31 has beenformed is referred to as disc substrate of each of examples.

First of all, as the ultraviolet ray setting resin 32 to be coated, witha resin that does not contain a solvent and whose viscosity is 4.0×10⁻²Pa·s (40 cps), the reaction protection resin layer 31 has been formed onthe replica substrate 1 a. As a result, the disc substrate 1 has beenproduced. At that point, the non-solvent type resin has been supplied ata radial position of 17 mm of the replica substrate 1 a. The discsubstrate 1 has been rotated at 83.3 s⁻¹ (5000 rpm) about the rotatingshaft. A disc substrate produced in a rotation time of 1 sec is referredto as disc substrate 1 of Example 11. A disc substrate produced in arotation time of 2 sec is referred to as disc substrate 1 of Example 12.A disc substrate produced in a rotation time of 4 sec is referred to asdisc substrate 1 of Example 13. A disc substrate produced in a rotationtime of 7 sec is referred to as disc substrate 1 of Example 14. A discsubstrate produced in a rotation time of 10 sec is referred to as discsubstrate 1 of Example 15. A disc substrate produced in a rotation timeof 20 sec is referred to as disc substrate 1 of Example 16.

In addition, as the ultraviolet ray setting resin 32 to be coated, witha solvent type resin that contains 50 weight % of methoxy propanol andthat has a viscosity of 1.0×10⁻¹ Pa·s (10 cps), the reaction protectionresin layer 31 has been formed on the replica substrate 1 a. They havebeen left for 30 seconds. After the solve had been fully given off, theyhave been hardened by the radiation of an ultraviolet ray. As a result,the disc substrate 1 has been produced. The solvent type resin has beensupplied at a radius position of 17 mm of the replica substrate 1 a. Thedisc substrate 1 has been rotated about the center of the rotating shaftin a rotation time of 83.3 s⁻¹ (5000 rpm). A disc substrate produced ina rotation time of 1 sec is referred to as disc substrate 1 of Example17. A disc substrate produced in a rotation time of 2 sec is referred toas disc substrate 1 of Example 18. A disc substrate produced in arotation time of 4 sec is referred to as disc substrate 1 of Example 19.A disc substrate produced in a rotation time of 7 sec is referred to asdisc substrate 1 of Example 20. A disc substrate produced in a rotationtime of 10 sec is referred to as disc substrate 1 of Example 21. A discsubstrate produced in a rotation time of 20 sec is referred to as a discsubstrate 1 of Example 22.

Table 3 shows film thickness characteristics of the reaction protectionresin layer 31 of the disc substrates 1 of Example 11 to Example 16produced with the aforementioned non-solvent type ultraviolet raysetting resin 32 (namely, the average film thickness of the reactionprotection resin layer 31 in the data region of the disc substrates 1,the film thickness of the reaction protection resin layer 31 in theinner circumference of the data region (radius: 24 mm), the filmthickness of the reaction protection resin layer 31 in the outercircumference of the data region (radius: 58 mm), and the differencebetween the inner and outer circumferential film thicknesses. Table 4shows the results of the film thickness characteristics of the reactionprotection resin layer 31 of the disc substrates 1 of Example 17 toExample 22 produced with the ultraviolet ray setting resin 32, whichcontains a solvent.

TABLE 3 Example Example Example Example Example Example 11 12 13 14 1516 Rotation time[s] 1 2 4 7 10 20 Average film thickness 7.6 5.8 4.3 3.02.7 1.9 [μm] Difference between 2.3 1.8 1.3 1.0 0.8 0.6 inner and outercircumferential film thickness [μm]

TABLE 4 Example Example Example Example Example Example 17 18 19 20 2122 Rotation time[s] 1 2 4 7 10 20 Average film thickness 2.9 2.2 2.0 1.81.7 1.6 [μm] Difference between 0.4 0.3 0.2 0.2 0.1 0.1 inner and outercircumferential film thickness [μm]

Table 3 shows that when the reaction protection resin layer 31 is formedwith the non-solvent type ultraviolet ray setting resin 32 by the spincoat method, as the rotation time is increased, the average filmthickness and the difference between the inner and outer circumferentialfilm thicknesses are decreased. Table 4 shows that when the reactionprotection resin layer 31 is formed with the ultraviolet ray settingresin 32 that contains a solvent by the spin coat method, as therotation time is increased, the average film thickness is decreased andthe difference between inner and outer circumferential film thicknessesbecomes very large in comparison with those of the disc substrates 1 ofExample 11 to Example 16. When the ultraviolet ray setting resin isformed by the spin coat method, the film thickness of the innercircumferential portion tends to be larger than the film thickness ofthe outer circumferential portion. Thus, it is preferred to use a resinthat contains a solvent as the ultraviolet ray setting resin 32. Withthe ultraviolet ray setting resin that contains the solvent, it is clearthat the difference between the inner and outer circumferential portionsof the data area becomes 1 μm or smaller.

As described above, after the reaction protection resin layer 31 hasbeen formed, the disc substrate 1 on which the reaction protection resinlayer 31 has been formed and the sheet 4 of the first embodiment areadhered by the adhering device of the first embodiment. As a result, theoptical disc according to the third embodiment has been produced.

The inventors of the present invention have measured the initialreflectance and the reflectance after the acceleration tests for theoptical discs of which the sheet 4 has been adhered to the front surfaceof the reaction protection resin layer 31 of the disc substrates 1 ofExample 11 to Example 22 (these optical discs are referred to as opticaldiscs of Example 11 to Example 22) in the same manner as the firstembodiment and calculated the rates of variation of reflectance thereof.An optical disc of Comparative Example is the same as that of the firstembodiment.

TABLE 5 Comparative Example Example Example Example Example Exampleexample 11 12 13 14 15 16 Film thickness of 0 7.6 5.8 4.3 3.0 2.7 1.9reaction protection resin layer [μm] Initial reflectance [%] 20.0 18.818.9 19.1 19.3 19.5 19.5 Reflectance after 14.3 18.5 18.5 18.6 18.7 18.918.8 acceleration test [%] Amount of variation of 5.7 0.3 0.4 0.5 0.50.6 0.7 reflectance [%] Amount of variation of 0.285 0.016 0.021 0.0260.026 0.031 0.036 reflefctance/ Initail reflectance

TABLE 6 Comparative Example Example Example Example Example Exampleexample 17 18 19 20 21 22 Film thickness of 0 2.9 2.2 2.0 1.8 1.7 1.6reaction protection resin layer [μm] Initial reflectance [%] 20.0 19.319.3 19.4 19.5 19.5 19.6 Reflectance after 14.3 18.8 18.8 18.8 18.9 18.818.8 acceleration test [%] Amount of variation of 5.7 0.5 0.5 0.6 0.60.7 0.8 reflectance [%] Amount of variation of 0.285 0.026 0.026 0.0310.031 0.036 0.041 reflectance/ Initail reflectance

Table 5 shows that the rate of variation of reflectance of the opticaldisc of Comparative Example is 0.285 and that when at least the reactionprotection resin layers 31 have been formed on the optical discs ofExample 11 to Example 16, the rates of variation of reflectance thereofcan be decreased. In the optical disc of Comparative Example, yellowishdiscoloring has been observed. In the optical discs of Example 11 toExample 16, discoloring of the adhesive layer 2 b has been hardlyobserved. The adhesive layer 2 b has been transparent.

When the rate of variation of reflectance exceeds 0.1, therecording/reproducing characteristics of an information signaldeteriorate. It is clear that the rate of variation of reflectance ofthe optical disc of Example 16 is 0.036, which is very small, althoughthe film thickness of the reaction protection resin layer 31 of Example16 is 1.9 μm, which is the smallest in the optical discs of Example 11to Example 16. Thus, it is clear that when the reaction protection resinlayer 31 is formed, it is certain that the rate of variation ofreflectance becomes 0.1 or smaller and thereby goodrecording/reproducing characteristics can be obtained.

Table 6 shows that the rate of variation of reflectance of the opticaldisc of Comparative Example is 0.285 and that when at least reactionprotection resin layer 31 has been formed, the rate of variation ofreflectance of the optical discs of Example 17 to Example 22 can bedecreased. In the optical disc of Comparative Example, yellowishdiscoloring has been observed. However, in the optical discs of Example17 to Example 22, discoloring of the adhesive layer 2 b of the opticaldiscs has been hardly observed. The adhesive layer 2 b has beentransparent.

It is clear that the rate of variation of reflectance of the opticaldisc of Example 22 is 0.041, which is very small, although the filmthickness of the reaction protection resin layer 31 of Example 22 is 1.6μm, which is the smallest in the optical discs of Example 17 to Example22. Thus, it is clear that when the reaction protection resin layer 31is formed, it is certain that the rate of variation of reflectancebecomes 0.1 or smaller and thereby good recording/reproducingcharacteristics can be obtained.

In the optical disc according to the third embodiment of the presentinvention, since the ultraviolet ray setting resin is coated andhardened on the replica substrate 1 a so that the ultraviolet raysetting resin fully coats the information signal portion 1 c. As aresult, the reaction protection resin layer 31 is formed. The lighttransmission layer 2 is formed through the reaction protection resinlayer 31. As a result, the same effect as the first embodiment can beobtained.

When the optical disc according to the third embodiment is produced, theultraviolet ray setting resin 32 that contains a solvent is coated onthe reaction protection resin layer 31 by the spin coat method. Thus,the light transmission layer of which the difference between the innerand outer circumferential film thicknesses of the data region is smallcan be formed. Consequently, an optical disc having goodrecording/reproducing characteristics can be produced.

So far, embodiments of the present invention have been practicallydescribed. However, the present invention is not limited to those.Instead, there are various modifications of the present inventionwithout departing from the spirit thereof.

For example, numerical values, materials, structures of optical discsthat have been described in the forgoing embodiments are just examples.When necessary, different numerical values, materials, and structures ofoptical discs may be used.

In the forgoing embodiments, the present invention is applied to opticaldiscs having a light transmission layer. In addition, the presentinvention can be applied to optical hard disks and removable opticalhard disks that use magneto-optical recording and reproducing. Accordingto the forgoing first embodiment, the present invention is applied to aphase change type optical disc, where an information signal is recordedusing a phase change. However, the present invention can be applied toother rewritable optical disc, a write once type optical disc, or a readonly type optical disc without departing from the spirit of the presentinvention.

When two optical discs of an embodiment of the present invention areadhered so that their light transmission layers 2 face outside, adouble-sided optical disc can be produced.

In addition, according to the first embodiment, an example of which aninformation signal portion is formed on a substrate has been described.Alternatively, the information signal portion may be formed on a sheetso that the information signal portion faces the substrate. The sheetmay be made of a plurality of thin films. The outermost layer may beunevenly formed as the information signal portion.

As described above, according to the present invention, since a reactionprotection layer is formed on an adhesive layer side of the informationsignal portion, the information signal portion on one main plane of thesubstrate can be prevented from chemically reacting with the adhesivelayer of the light transmission layer. Thus, in the optical disc ofwhich the light transmissivity sheet has been adhered to one mainsurface of the substrate through the adhesive layer, the variation ofreflectance of each optical disc can be suppressed. In addition, thevariation of reflectance of the recording/reproducing plane of theoptical disc can be suppressed. Moreover, the optical disc can conformwith a large NA of an objective lens used to record/reproduce data. Theoptical disc has a light transmission layer that has small doublerefraction, high transparency, and equal film thickness. In addition,the production yield of the optical disc can be improved.

1. An optical disc comprising: a substrate; an information signalportion having a plurality of layers and configured to record and/orreproduce an information signal; and a light transmission layerconfigured to transmit laser light used to record and/or reproduce theinformation signal, the information signal portion and the lighttransmission layer being formed on one main plane of the substrate ofthe optical disc, wherein the light transmission layer comprises a sheethaving light transmissivity and an adhesive layer made of apressure-sensitive adhering agent for adhering the sheet to the mainplane of the substrate, wherein the light transmission layer has a filmthickness in the range from 90 μm to 110 μm, and wherein a reactionprotection layer having a dielectric of one of silicon nitride andsilicon oxide is formed on the information signal portion so that thereaction protection layer faces the adhesive layer.
 2. The optical discas set forth in claim 1, wherein the reaction protection layer has athickness in the range from 2 nm to 30 nm.
 3. The optical disc as setforth in claim 1, wherein the information signal portion has a filmthickness in the range from 183 nm to 313 nm.
 4. An optical disccomprising: a substrate; an information signal portion having aplurality of layers and configured to record and/or reproduce aninformation signal; and a light transmission layer configured totransmit laser light used to record and/or reproduce the informationsignal, the information signal portion and the light transmission layerbeing formed on one main plane of the substrate of the optical disc,wherein the light transmission layer comprises at least a sheet havinglight transmissivity and an adhesive layer configured to adhere thesheet to the main plane of the substrate, wherein the light transmissionlayer has a film thickness in the range from 90 μm to 110 μm, andwherein a reaction protection layer having an ultraviolet ray settingresin is formed between the information signal portion and the adhesivelayer.
 5. The optical disc as set forth in claim 4, wherein adistribution of the film thickness of the reaction protection layer in aregion of at least the information signal portion is 1 μm or smaller. 6.The optical disc as set forth in claim 4, wherein the information signalportion has a film thickness in the range from 183 nm to 313 nm.
 7. Theoptical disc as set forth in claim 4, wherein the reaction protectionlayer is formed by coating ultraviolet ray setting resin that contains asolvent on the information signal portion by a spin coat method, suchthat the solvent that is contained in the ultraviolet ray setting resinis substantially removed, and by hardening the ultraviolet ray settingresin.
 8. The optical disc as set forth in claim 7, wherein the solventis methoxypropanol.
 9. An optical disc, comprising: a substrate; aninformation signal portion configured to record and/or reproduce aninformation signal; and a light transmission layer configured totransmit laser light used to record and/or reproduce the informationsignal, the information signal portion and the light transmission layerbeing formed on one main plane of the substrate, wherein the lighttransmission layer comprises a sheet having light transmissivity and anadhesive layer configured to adhere the sheet to the main plane of thesubstrate, wherein the information signal portion comprises a reflectionlayer configured to reflect the laser light, a first dielectric layer, arecording layer configured to record the information signal, and asecond dielectric layer successively formed from the substrate, andwherein the second dielectric layer is made of a mixture of zinc sulfideand silicon oxide, and has a film thickness in one of the ranges from 45nm to 90 nm and from 130 nm to 175 nm, so that the reflectance of thelaser light on a flat plane of the substrate is 15% or larger.
 10. Anoptical disc producing method, comprising: forming an information signalportion configured to record and/or reproduce an information signal on amain plane of a substrate; and forming a light transmissivity layer overthe information signal portion, the light transmissivity sheet beingconfigured to transmit light used to record and/or reproduce theinformation signal in a region that covers the information signalportion through an adhesive layer, wherein the adhesive layer is made ofa pressure-sensitive adhering agent, wherein the light transmissionlayer has a film thickness in the range from 90 μm to 110 μm, andwherein a reaction protection layer having a dielectric of one ofsilicon oxide and silicon nitride is formed as an outermost layer of theinformation signal portion.
 11. The optical disc producing method as setforth in claim 10, wherein the reaction protection layer has a filmthickness in the range from 2 nm to 30 nm.
 12. The optical discproducing method as set forth in claim 10, wherein the informationsignal portion has a film thickness in the range from 183 nm to 313 nm.13. An optical disc producing method, comprising the steps of: formingan information signal portion configured to record and/or reproduce aninformation signal on a main plane of a substrate; forming a reactionprotection layer made of an ultraviolet ray setting resin on an upperlayer of the information signal portion; and forming a lighttransmissivity sheet configured to transmit laser light used to recordand/or reproduce the information signal in a region that covers theinformation signal portion through an adhesive layer, wherein the lighttransmission layer has a film thickness in the range from 90 μm to 110μm.
 14. The optical disc producing method as set forth in claim 13,wherein a distribution of the film thickness of the reaction protectionlayer in a region of at least the information signal portion is 1 μm orsmaller.
 15. The optical disc producing method as set forth in claim 13,wherein the information signal portion has a film thickness in the rangefrom 183 nm to 313 nm.
 16. The optical disc producing method as setforth in claim 13, wherein the reaction protection layer is formed bycoating ultraviolet ray setting resin that contains a solvent on theinformation signal portion by a spin coat method, so that the solventthat is contained in the ultraviolet ray setting resin is substantiallyremoved, and by hardening the ultraviolet ray setting resin.
 17. Theoptical disc producing method as set forth in claim 16, wherein thesolvent is methoxypropanol.